1. Field of the Invention
The present invention relates generally to micro-electromechanical (MEMS) devices and, in particular, to arrayed magnetically actuated MEMS devices such as arrayed mirrors used in optical switches.
2. Description of Related Art
FIG. 1 schematically illustrates an example of an optical cross-connect 12 of an optical switch. The cross-connect 12 includes an array of collimators or other beam-forming devices, represented by grid 14, and forms incoming optical communications signals into beams that impinge on an array of selectively moveable reflectors or mirrors represented by grid 16. Each beam from grid 14 has its own corresponding moveable mirror on grid 16.
The moveable mirrors of grid 16 are controllably positioned so as to individually direct the respective beams from grid 14 to respective moveable mirrors of a second array of moveable mirrors, represented by grid 18. The moveable mirrors of grid 18 are positioned so as to individually direct the beams received from grid 16 to respective beam receivers of an array of beam receivers represented by grid 20. The beam receivers may take various forms, such as transducers, lenses or optical elements for coupling the respective beams into respective optical fibers, waveguides, or the like. As with grids 14 and 16, each moveable mirror of grid 18 is associated with a particular beam receiver of grid 20, so that each receiver receives beams on a single axis. A representative signal path from grid 14 to grid 20 is indicated by arrow 22.
Attempts have been made previously to fabricate arrays of mirrors such as those represented by grids 16 and 18 using MEMS technology, in which silicon processing and related techniques common to the semiconductor industry are used to form micro-mechanical devices. For switches such as that shown in FIG. 1, it is desirable to have an array of moveable mirrors that are both densely packed and easily controlled.
As is known in the art, movable mirrors can be actuated or controlled in a variety of ways including through electromagnetic actuation, electrostatic actuation, piezoelectric actuation, stepper motors, thermal bimorph and comb-drive actuation.
FIG. 2 illustrates an electro-magnetically actuated single-mirror device 30 in accordance with the prior art. The mirror device 30 includes a mirror 32 movably supported on a gimbal structure 34. The mirror 32 includes a reflective surface 33, which is on the same side of the mirror as the actuation coils.
The device 30 has an inner coil 36 on the mirror 32, and an outer coil 38 on a gimbal frame. An external magnetic field B oriented at 45 degrees to the X and Y axes provides torque when either the inner or outer coils are actuated with current, thereby causing the mirror 32 to rotate about respective torsional hinges or flexures 40, 41 as desired.
The mirror device 30 accordingly has two axes of actuation (about the inner and outer hinges 40, 41) that are non-orthogonal to the applied magnetic field. Non-orthogonal actuation consumes greater power (or requires stronger magnetic fields) since the coil torques interfere with each other. This also complicates control of the device. In addition, the coils require space and reduce the area available for the mirror. Smaller mirrors cannot intercept as much of the desired optical beam, causing higher insertion loss. Alternatively, larger mirrors can be used, but with reduced packing density.
The magnetic field applied to mirror devices of the type shown in FIG. 2 is provided in the prior art by magnets positioned in the plane of the mirror. As shown, e.g., in FIG. 3, a mirror device 42 includes magnets 44 in the plane of the mirror 32. A strong magnetic field is needed at the plane of the mirror and gimbal to minimize the current needed to deflect the mirror, thereby reducing power consumption and heating of the mirror. The magnets 44 are accordingly relatively large. A frame 46 of soft magnetic material can be provided to intensify the field. The relatively large magnets and frame make it difficult to have multiple mirrors of this type positioned close to each other in an array.
Other known mirror actuators also have drawbacks. For example, currently available electrostatic devices require large voltages and have a nonlinear rotation angle vs. applied voltage relationship. In addition, small gaps are needed in these devices, which can become clogged with particles. Also, electrostatic actuation currently provides only very weak forces for large displacements or strong forces for only small displacements, whereas electromagnetic actuation can produce large forces over large displacements.
Piezoelectric actuators have very small deflection angles or exert extremely small forces (bimorphs) or both. Stepper motors are very large and typically consume more power. Thermal actuators have a slow response time, are sensitive to ambient temperature, are energy inefficient and consume large amounts of power. Heat spreading from adjacent devices causes thermal cross-talk.
A need accordingly exists for an array of mirror devices that can be densely packed and easily controlled.
A need also exists for a mechanism that can be used to detect the angular position of a movable mirror.
A need further exists for improved packaging of mirror device arrays.
A need also exists for an improved method of manufacturing mirror devices.